1. Field of the Invention
The present invention concerns an x-ray CT system (computed tomography) for x-ray phase contrast and/or x-ray dark field imaging of a scanned examination subject.
2. Description of the Prior Art
CT systems for x-ray phase contrast and/or dark field imaging of a subject are known that have at least one grating interferometer arranged at a gantry, the at least one grating interferometer having first, second and third grating structures:
The first grating structure has a number of band-shaped x-ray emission maxima and minima arranged in parallel, the maxima and minima have a first grating period. The second grating structure produces, as a phase grating, a partial phase offset of x-ray radiation passing therethrough and exhibits a second grating period. The third grating structure has a third grating period with which relative phase shifts of adjacent x-rays and/or scatter components are detected. The three grating structures, with regard to their distances from one another and at least the first and second grating structure with regard to their grating periods, satisfy the Talbot conditions.
The known CT systems also have a device for value-based determination of the phase between adjacent x-rays and/or for value-based determination of the spatial intensity curve per detector element perpendicular to the bands of the grating structures.
Such x-ray CT systems for x-ray phase contrast and/or x-ray dark field imaging of a scanned examination subject are known from EP 1 731 099 A1, EP 1 803 398 A1 and DE 10 2006 017 290 A1 for example.
The use of x-ray-optical gratings allows the acquisition of x-ray images in phase contrast, which x-ray images deliver additional information about an examination subject and/or enable a smaller x-ray dose given the same image contrast. The possibility also exists for not only the phase information, but also the amplitude information of scattered radiation, to be used for imaging. An image can be generated that is based exclusively on the scatter components of the x-ray radiation diffracted by the examination subject, thus a least angle scattering. Very slight density differences in the examination subject then can be shown at very high resolution. The publication from F. Pfeiffer et al., “Hard X-ray dark-field imaging using a grating interferometer”, Nature Materials 7, pp 134-137 is referenced in this regard.
In order to obtain this desired information of an examination subject irradiated with incoherent radiation from x-ray tubes under practical conditions, three grating structures must be used whose periods lie approximately in the range from 1 to 100 micrometers. The webs of the medium grating structure—the analysis grating—are formed of phase-shifting material and generate a phase shift of π or π/2 according to T. Weitkamp et al.: Proc. SPIE 6318, Developments in X-Ray Tomography V (2006) p. 6318-28. The two other grating structures generally are fashioned as absorption gratings with webs fashioned from absorbing material with the highest possible absorption.
For examinations in which the phase differences between adjacent beams have actually been analytically detected and determined, or in which not only the phase information but also the amplitude information have been analytically determined per pixel at detectors, an arrangement has conventionally been selected in which the distance l between the first and second grating structures G0 and G1 is greater than the distance d between the second and third grating structures G1 and G2. The sample or the gantry opening is arranged between the first and second grating structures G0 and G1. This arrangement results in the corresponding grating periods being p0>p1>p2. Particularly the technical realization of the grating structure G2 with absorber structures has proven to be problematic since the smallest grating period p2 and the grating lines must have a high absorption. This requires the use of highly absorbent materials such as gold. At the same time, the area of G2 is the largest of all three gratings, which also requires a significant quantity of expensive gold in addition to the production cost.
In FIG. 5 of U.S. Pat. No. 5,812,629, a CT system with a grating interferometer is shown in which the examination subject is arranged between the second and third gratings, wherein the distance between the first two gratings is smaller than the distance between the last two gratings. In this embodiment of the disclosed CT system, however, a value-based analysis of the spatial intensity curve is not implemented per detector element, and thus the phase and amplitude of this intensity curve are also not determined analytically.
In U.S. Pat. No. 7,180,979 B2, an arrangement is disclosed in which the examination subject is positioned between the second and third gratings; but in this embodiment of the CT system a value-based analysis of the spatial intensity curve is not implemented for each detector element, and thus the phase and amplitude of this intensity curve are also not determined analytically.
Furthermore, in published Patent Application WO 2007/12533 A1, a CT system for value-based determination of phase shifts with a Talbot interferometer is proposed in which the grating periods increase in the beam direction, wherein the examination subject can be placed between the second grating structure and third grating structure; however, the ratios of the grating periods relative to one another and the ratios of the intervals between the gratings that are proposed there are unsuitable in practice with a CT system.